Single-photon avalanche diodes (SPAD), also called Geiger-mode avalanche photodiodes, are suitable to detect extremely weak radiation. Avalanche photodiodes having the pn junction biased in the reverse direction become conducting when the applied voltage exceeds a breakdown voltage. A single-photon avalanche diode is operated with reverse bias voltage above the breakdown voltage, in the so-called Geiger-mode. In this mode of operation the electric field strength in the space charge region can attain values of more than 500 kV/cm, so that already few electron-hole pairs generated by incident photons and injected into the depletion layer can trigger a self-sustaining avalanche current of several mA.
US 2012/0025340 A1 discloses a vertical silicon photomultiplier for Geiger-mode operation. A pattern of trench electrodes provides anodes and cathodes of a pixel array. A pn-junction layer is arranged at the walls of the cathodes.
US 2010/0148040 A1 discloses a single-photon avalanche diode comprising an n-type semiconductor body providing an active region including the cathode, a trench extending through the body around the active region and providing a laterally isolating region, a depletion region, and a p-type anode region at a surface of the active region.
U.S. Pat. No. 7,462,889 B2 discloses an avalanche photodiode with a ring shape trench region surrounding a light receiving region.
US 2008/0191240 A1 discloses an avalanche photodiode with an outer trench provided in the periphery of a light absorption layer surrounding the depletion region to interrupt the path of the dark current.
US 2007/0145411 A1 discloses a trench polysilicon diode.
For at least 30% of incoming radiation to be detected in the near infrared spectrum, at the wavelength of 800 nm, for example, the absorption region has to be about 7 μm thick. A fast response of the photodiode is achieved with a wide space charge region, which can unfortunately only be obtained in conventional single-photon avalanche diodes if the breakdown voltage of the pn-junction is increased to values much higher than the range from 20 V to 40 V of the voltages that are preferably applied. If the breakdown voltage is sufficiently low to allow Geiger-mode operation even in this low voltage range, the dimension of the space charge region is limited to the range from ca. 0.7 μm to ca. 1.5 μm, and only about 10% of the radiation having the wavelength of 800 nm is absorbed.